Underground oil recovery from solid oil-bearing deposits



M. PRATS Sept. 19, 1967 BEARING DQEPOSITS UNDERGROUND OIL RECOVERY FROM SOLID OIL- Filed March 6, ,1964

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INVENTOR:

M. ,PRATS BY= H1 HIS A'GENT United States Patent 3,342,258 UNDERGROUND OIL RECOVERY FROM SOLID OIL-BEARING DEPOSITS Michael Prats, Houston, Tex., assignor to Shell Oil Company, New York, N.Y., a corporation of Delaware Filed Mar. 6, 1964, Ser. No. 349,923 4 Claims. (Cl. 166-11) This invention relates to the art of recovering oil from underground formations, and pertains more particularly to oil recovery methods which utilize combustion within the reservoir to aid in the displacement and recovery of the oil. In one of its more specific embodiments, the invention is directed to the recovery of oil from solid oilbearing deposits, such as the Athabasca tar sands and the like or the large deposits of oil shale found in various sections of the United States, particularly in Colorado and surrounding States.

Various techniques have been tried in an attempt to recover oil, particularly from bituminous or solid oilbearing deposits and tar sands. However, many of them have been found to be unsuccessful for one or more reasons. Thus, one of the more often-attempted methods includes fracturing a bituminous formation to open passages therein between at least two wells, introducing a propping agent in the open passages or fractures in an attempt to maintain them permeable when the pressure on the formation has been decreased so that the fractures close, and then trying to initiate and maintain combustion so as to volatilize at least a portion of the oil and attempt to displace it to a production well or wells in order to effect the recovery of the oil therefrom.

There are certain defects and disadvantages which are inherent when the above-outlined technique is used for recovery of oil, for example, from tar sands and the like. Fractures are formed by increasing the pressure on a fluid confined within a section of a borehole. The direction along which the fracture is most extensive is determined by the tectonics and cannot to any significant extent be controlled by personnel conducting the fracturing operation. The fracture may be vertical and extensive along only, for example, a north-south direction. On the other hand, it may be horizontal and radial but have a significant thickness and extent only in one direction.

Field experience has demonstrated that in transporting a granular or propping agent such as sand into a fracture, the propping agent may move away from the injection well along a meandering path. For example, in fracturing a well surrounded by concentric rings of wells, the propping agent may be transported to both the near and far wells north of the injection well, to the far but not the near wells west of the injection well, and may never be transported to any of the wells south or east of the injection well.

Thus, it may be seen that there are certain inherent disadvantages in fracturing a formation and propping the fracture with, for example, sand to improve the overall permeability between a pair of wells. Sand can be strained out of the fracturing fluid entering a thin portion of a fracture. A horizontal and radially-extensive fracture may be thick enough to accommodate the passage of sand grains in some sectors but not in others. For example, when the sand-transporting fluid is injected, the sand grains may move into only a northeast sector since in other directions the fracture is too thin to permit the passage of sand grains. In such a case, when the fracturing pressure is reduced and the formation is allowed to subside, a permeable streak will have been established only in the northeast sector.

Additionally, sand can be dropped out of a fracturing fluid moving at a low velocity. A horizontal and radially- Patented Sept. 19, 1967 extensive fracture having a uniform thickness adequate for the passage of said grains may receive the sand or propping agent only near the well. As the sand-transporting fluid is pumped into such a fracture at a given rate, the volume of the space into which the fluid is moving increases with distance away from the well, and the velocity at which the fluid is moving is correspondingly decreased. When the velocity of a sand-transporting fluid reaches the minimum value unique to the particular fluid, a sand out occurs as the fluid deposits the grains it was transporting. Therefore, the injection of a sand-transporting fluid into such a fracture may result in the establisment of a permeable streak in the immediate vicinity of the injection Well.

In oil fields wherein the oil-bearing formation contains viscous petroleum materials, the forming and propping of a fracture with sand is, in practice, ineflective as a means of improving fluid communication between wells. Unless such a fracture can be preheated and maintained hot, it is promptly plugged by the intrusion of the viscous petroleum material. It is to be further noted that hydraulicallyinduced fractures have thicknesses only in the order of A; to A of an inch. Even where fractures can be propped with sand grains to maintain most of this thickness after fracturing pressure has been reduced, the sand-filled fractures only amount to very thin streaks of high permeability.

It is therefore an object of the present invention to avoid the above and other defects and disadvantages and to provide an economical and eflicient method for recovery of hydrocarbones from oil-bearing under round formations, and particularly from bituminous deposits.

A further object of the present invention is to provide a recovery method wherein the permeability between pairs of wells can be established or substantially increased without the need for fracturing the oil-bearing formation and propping the fractures with sand particles.

Another object of the present invention is to provide a method for recovering oil from underground oil-bearing formations which process utilizes in situ combustion, which makes use of a zone of increased permeability established between pairs of wells.

These and other objects of this invention will be understood from the following description taken with reference to the drawing, wherein:

FIGURE 1 is a diagrammatic view taken in longitudinal section of three wells in which a fracturing operation is being carried out in order to establish communication through an oil-bearing formation between the wells; and,

FIGURE 2 is a diagrammatic view taken in partial longitudinal section illustrating the three wells of FIG- URE 1 at a time when the fracturing pressure has been reduced and after a permeable zone has been established between the Wells.

The oil recovery method of the present invention contemplates the use of at least a pair of wells extending down into communication with an oil-bearing formation and preferably additional Wells whereby in situ combustion or a fluid drive may be carried out. With the formation to be treated being provided with at least a pair of wells, the oil-bearing formation is then hydraulically fractured, preferably with horizontal fractures extending the entire way between the wells and at a proper vertical level or levels within the fomation and subsequently an oilremoval or entraining fluid is conveyed through the fracture or fractures thus formed to extract oil from the fractures as well as the portions of the formation adjacent thereto to form a permeable flow area between the wells. After a permeable flow area has been formed in this manner, combustion is initiated along the resultant and more permeable zone of the oil-bearing formation and oil is produced by advancing the combustion zone toward one or more producing wells.

Referring to FIGURE 1 of the drawing, three wells are shown as having been drilled down into the oil-bearing zone and aligned with casings 10, 11 and 12. The casings 10, 11 and 12 are preferably closed at the top, depending upon the operation being carried out, with smaller-diameter pipes 13, 14 and 15 extending through the closed upper ends of the casings 10, 11 and 12. The smaller-diameter pipes 13, 14 and 15 may be used for various operations, depending on how they are equipped or what they are connected to. Thus, in the operation shown in FIGURE 1 at least one of the pipes, say pipe 13, would be connected to a source of pressure fluid whereby the oil-bearing formation 16 could be fractured, as at 17, with the fracture extending between at least two of the wells. The portions of the well casings 11 and 12 Within the oil-bearing formation 16 are provided with series of perforations 18 whereby the interior of each of the well casings is brought into communication with the oil-bearing zone whereby fluids can be either forced into the zone 16 or extracted therefrom.

The wells in the oil field may be newly drilled wells or may have been wells from which production fluid is no longer obtained. If the wells had not been previously drilled and it is necessary to drill new wells, the wells are preferably placed in a standard, staggered, spaced pattern with relatively close spacing between them.

After the wells have been established in a suitable position, the oil-bearing formation is fractured, preferably in a generally horizontal plane whereby the hydraulicallyformed fractures extend all the way between wells at the proper vertical levels within the formation. It is to be understood that at some levels vertical fractures will be formed instead and these vertical fractures can also be utilized by the method of the present invention. Generally, it is preferable to have the fractures disposed near the bottom of the oil-bearing formation. However, the fractures may be located elsewhere, and it is also desirable at times to fracture the oil-bearing formation or stratum at one or more levels so as to facilitate the in situ combustion in the oil-bearing formation as well as the recovery of oil therefrom.

The fractures 17 may be held open by fluid under pressure, said fracture fluid being preferably an oil-removing or entraining fluid which is passed from one well to another to leach out the oil in the fractures and in portions of the formation forming the walls of the fracture. Any of the well-known oil-entraining or removal fluids may be used for this purpose. Thus, it is possible to employ normally gaseous hydrocarbon fractions, such as methane, ethane, propane, butane and pentane hydrocarbons and mixtures thereof, which latter may be used in a liquid, gaseous or mixed phase state. Also, instead of using the hydrocarbon fractions, which are miscible with the oil to be entrained and recovered from the fractures and the formations immediately adjacent thereto, it is also possible to use non-miscible entraining or removal fluids. Such non-miscible entraining fluids may be aqueous solutions containing a surfactant, e.g., an alkali metal soap or a non-ionic surfactant. These surfactants may be added as such to the water introduced into the formation to entrain or leach the oil therefrom, or, as an alternative, the surfactant may be formed in situ, e.g., by first introducing one reagent such as an acid-action material (such as a fatty acid), into the formation, and following it with an aqueous solution containing a second reagent, e.g., a basic-acting compound, such as sodium hydroxide so that the reagents interact in the fractures and surrounding formations to form a surfactant which aids entraining or leaching the oil, and aiding in its recovery. As stated, such a treatment opens up a formation and helps in the subsequent step of recovering the oil from the formation thus treated.

After the establishment of the horizontal passageways and having the walls of the passageways leached so as to remove a substantial amount of the oil originally present therein, it will be found that a zone of increased permeability has been formed between the wells which exists after the fracture has been allowed to close by the reduction of the pressure of the injected fluid. Use is subsequently made of this permeable zone between the wells to carry out any standard pattern for recovering oil from the oil-bearing formation, such, for example, by in situ combustion.

It is a uniquely advantageous feature of this invention that by selecting the locations and rates at which the oilremoval fluids are pumped into and out of the oilbearing formation, the zones of increased permeability can be shaped into the patterns best suited for the subsequent production by in situ combustion. For example, from wells arranged in rows the entraining fluids can be forced to flow along continuous paths directly across the space between the rows to provide a substantially uniform permeable zone extending like a blanket beneath a series of rows of wells to be utilized in a line drive underground combustion oil production process. When a zone of adequate permeability has been provided between a 'pair of wells, it is preferable but not essential to reduce the pumping pressures and allow the formation to sink down onto the channel of cleaned formation, that is, the zone of increased permeability 2% (FIGURE 2). In addition to reducing the operating costs during the in situ combustion production phase, this lowering of the pumping pressure tends to limit the extensions of the channel of clean sand and prevent the channel from being extended beyond a desired production well. A zone of adequate permeability is established when a combustion-supporting gas well flows between a pair of wells at a rate required to support underground combustion in response to a pressure less than the overburden pressure. In general a flow of at least several hundred thousand cubic feet per day is desirable to support the combustion.

The initiation of combustion along the resulting permeable zone 20 of the oil bearing formation 16 is accomplished by pumping a combustion-supporting gas through the channel of clean sand and heating oil that is contacted by the gas to a temperature at which the oil is ignited. This can be facilitated by use of down-hole heaters and any suitable chemical and/ or mechanical means for increasing the speed or ease of igniting the oil in a subsurface formation. When down-hole heaters are used, the combustion-supporting gases can comprise air which is heated as it passes the heater. When steam is used in cleaning the formation in the previous step, it is particularly advantageous to use steam as a heat-imparting gas to assist in the ignition. During the combustion drive step, the pumping pressures are preferably, but not necessarily, held below the overburden pressure. In carrying out an underground combustion in the present method, the ignition occurs along the edges of the channel of clean sand in the vicinity of the injection well. As the oil is heated to a temperature at which it begins to flow, the oil moves into and along the channel of increased permeability under the impetus of the gas stream. In general, the gas tends to move away from the injection well in all directions and then is guided, confined and accelerated by the walls of the permeable channel.

A reverse drive combustion process, in which the oil is ignited near the production well and the combustion front is propagated against the flow of gas toward the injection well, comprises a preferred type of underground combustion process for use in the present invention. In this type of process the injected combustion-supporting gas is guided by the impermeable formation bounding the permeable channel and conveyed to the continuously expanding zone in which the heat is most efliciently utilized. In the hot zone, downstream from the combustion point the oil in the formation is concurrently heated and displaced to leave a zone that tinually expanding.

Thus, it has been seen that by the present invention a method has been provided for forming a permeable flow Zone between a pair of wells by leaching of the oil from the formation rather than using sand as a propping agent. Leaching avoids the problem of having sand straining out or depositing in a thin portion of a fracture thereby reducing the distance or limiting the direction along which a permeable streak is established. The leaching fluid creates permeability by dissolving plugging materials. Thus, permeability in a formation can be created wherever a leaching fluid can be caused to flow. The leaching fluid can be forced to flow in one or a plurality of directions between a pair or a series of wells which are to be used in an underground combustion operation by employing a pressure sufiicient to hydraulically support a fracture until the leaching action has established an adequate degree of permeability through the leached-out walls of the fracture.

Leaching also avoids the problem of having a sand out limit the distance over which a streak of permeability is established. The leaching fluid can be forced to flow along and dissolving plugging materials along the full extent of the distance between the wells which are to be used in a fluid drive operation. Where a viscous petroleum material tends to plug permeable zones, the leaching can be done with steam which heats while it extracts. In the later stages of leaching and heating a zone with steam, air can be mixed with the steam and the mixture can be used to ignite the petroleum present along the edges of the permeable zone while that zone is maintained hot. In leaching operations, the thickness of the permeable zone can be extended for a significant distance into the wall of the fracture. The leaching fluid removes the petroleum exposed to the fracture and then moves into the regions previously plugged by the presence of petroleum to progressively increase the thickness of the permeable zone.

From the above description it may be seen that the key aspects of the present invention involve the following steps. (1) Opening a pair of wells into an oil-bearing reservoir formation that is impermeable at the reservoir conditions but is permeable when the oil is heated or extracted. (2) Pumping liquid between the wells at pressures and rates adjusted to fracture the formation and maintain a separation between layers of the formation along a flow path extending between the wells in a selected pattern that is established by the locations and rates at which the liquid is pumped into and out of the reservoir formation. (3) Pumping an oil-entraining liquid along the same flow path and extracting oil from the adjacent formation layers until portions of the layers become permeable and a combustion-supporting gas will flow between the wells, at a rate supporting an underground combustion, in response to a pressure less than the overburden pressure. (4) Pumping a combustion-supporting gas between the wells, igniting oil that is contacted by the gas, and producing oil that is heated by the combustion.

This combination of steps makes it possible to shape the permeable channel in the manner best suited for the type of underground combustion process that is to be used. It also uses the formation characteristic of being impermeable when cold and permeable when hot to provide an expendible channel for guiding the injected air to the point at which it is reacted. For example, in a reverse combustion, substantially all of the air is confined within the permeable zone and conveyed to the combustion zone near the producing well. As the combustion heats the adjacent formation, the oil is displaced by hot combustion products and that portion of the formation is rendered permeable. Thus, the channel that conveys the combustion-supporting gas to the point at Which it is reacted is destroyed as the hot zone expands and creates a permeable is hot, permeable and conformation through which a discrete channel of permeability is no longer needed.

In the present process the normal impermeability of the formation is used (1) as part of the system for conveying the combustion-supporting gas to the combustion front and (2) as a means for shaping the permeable zone through which the leading edge of the combustion front will propagate to a shape best suited for the type of underground combustion drive that is to be performed.

I claim as my invention: 1. A method of recovering oil from an underground oil-bearing formation selected from the group consisting of tar sand and oil shale which is penetrated by a plurality of wells, at least one of said wells being normally a fluid injection well and at least one well adjacent said injection well being normally an oil-production well, said method comprising establishing communication between all of said wells and the oil-bearing formation surrounding said wells,

fracturing the formation between a fluid-injection well and at least one adjacent oil-production well in a manner such that the wells are in fluid flow communication with each other through said oil-bearing formation, injecting an oil-removal fluid down the injection well and through the fracture to said production well at a pressure sulficient to maintain the fracture open,

maintaining said injection of oil-removal fluid through said fracture for a time suflicient to remove oil from the walls of the formation forming the fracture and increase the permeability of said walls to a value sufiicient to permit the flow of oil therethrough when said fracture is closed,

stopping the injection of said oil-removal fluid and reducing the fluid injection pressure whereby said fracture closes with a permeable zone formed along said closed fracture line,

injecting a combustible driving oil-displacement fluid in said injection well and through said oil-bearing formation to said production well at a pressure less than that which would cause the formation between said wells to fracture, and

recovering oil therefrom through said oil-production well.

2. The method of claim 1 wherein said oil-bearing formation includes at least one layer of a permeable matrix which extends between the wells and is plugged by oil and wherein said fracturing operation is carried out at the level of said permeable matrix to fracture said matrix.

3. The method of claim 1 wherein the driving fluid is obtained by initiating combustion in said formation between said wells to drive oil to a production well.

4. The method of claim 3 wherein said combustion is accomplished by pumping combustion supporting gases down said injection well and through the previouslyformed permeable zone between the wells until oil along the edges of said zone is ignited.

References Cited UNITED STATES PATENTS CHARLES E. OCONNELL, Primary Examiner.

JACOB L. NACKENOFF, STEPHEN J. NOVOSAD,

Examiners. 

1. A METHOD OF RECOVERING OIL FROM AN UNDERGROUND OIL-BEARING FORMATION SELECTED FROM THE GROUP CONSISTING OF TAR SAND AND OIL SHALE WHICH IS PENETRATED BY A PLURALITY OF WELLS, AT LEAST ONE OF SAID WELLS BEING NORMALLY A FLUID INJECTION WELL AND AT LEAST ONE WELL ADJACENT SAID INJECTION WELL BEING NORMALLY AN OIL-PRODUCTION WELL, SAID METHOD COMPRISING. ESTABLISHING COMMUNICATION BETWEEN ALL OF SAID WELLS AND THE OIL-BEARING FORMATION SURROUNDING SAID WELLS, FRACTURING THE FORMATION BETWEEN A FULID-INJECTION WELL AND AT LEAST ONE ADJACENT OIL-PRODUCTION WELL IN A MANNER SUCH THAT THE WELLS ARE IN FLUID FLOW COMMUNICATION WITH EACH OTHER THROUGH SAID OIL-BEARING FORMATION, INJECTING AN OIL-REMOVAL FLUID DOWN THE INJECTION WELL AND THROUGH THE FRACTURE TO SAID PRODUCTION WELL AT A PRESSURE SUFFICIENT TO MAINTAIN THE FRACTURE OPEN, MAINTAINING SAID INJECTION OF OIL-REMOVAL FLUID THROUGH SAID FRACTURE FOR A TIME SUFFICIENT TO REMOVE OIL FROM THE WALLS OF THE FORMATION FORMING THE FRACTURE AND INCREASE THE PERMEABILITY OF SAID WALLS TO A VALUE SUFFICIENT TO PERMIT THE FLOW OF OIL THERETHROUGH WHEN SAID FRACTURE IS CLOSED, STOPPING THE INJECTION OF SAID OIL-REMOVAL FLUID AND REDUCING THE FLUID INJECTION PRESSURE WHEREBY SAID FRACTURE CLOSES WITH A PERMEABLE ZONE FORMED ALONG SAID CLOSED FRACTURE LINE, INJECTING A COMBUSTIBLE DRIVING OIL-DISPLACEMENT FLUID IN SAID INJECTION WELL AND THROUGH SAID OIL-BEARING FORMATION TO SAID PRODUCTION WELL AT A PRESSURE LESS THAN THAT WHICH WOULD CAUSE THE FORMATION BETWEEN SAID WELLS TO FRACTURE, AND RECOVERING OIL THEREFROM THROUGH SAID OIL-PRODUCTION WELL. 